1. Field of the Invention
The present invention generally relates to the file of disk drives adopting a perpendicular magnetic recording system, and more particularly to a write compensation facility when recording data on a disk.
2. Description of the Related Art
In recent years, in a disk drive field as typified by a hard disk drive, a disk drive adopting a perpendicular magnetic recording method has attracted attention as a technique for exceeding a limit in recording density in a longitudinal magnetic recording method.
In the perpendicular magnetic recording system, when digital data (0/1) is recorded on a disk which is a storage medium, a magnetization area corresponding to the data is formed in the perpendicular direction (depth direction). In the data read operation, an amplitude of a read signal waveform 51 output from a read head is, as shown in FIG. 5B varies at a magnetic transition position 52, and this signal has a rectangular signal waveform that the amplitude corresponds to the direction of magnetization.
Usually, in the disk drive adopting the perpendicular magnetic recording method, the read signal obtained by the read head is, as shown in FIG. 5C, converted into a differential waveform by a differential circuit. This differential waveform ideally has a maximum amplitude at the magnetic transition position 52. Thus, there can be obtained signals (1/0) having different amplitude polarities in accordance with transition from the differential waveform, from magnetization in the positive direction into magnetization in the negative direction, and from magnetization in the negative direction into magnetization in the positive direction.
In recent disk drives, there is adopted a magnetic head in which a read head and a write head are separately mounted on a slider. In the disk drive adopting the perpendicular magnetic recording method, the read head is usually a head consisting of a GMR (giant magnetoresistive) element, as similar to the drive for longitudinal magnetic recording.
On the other hand, the write head is usually a single pole type head.
Meanwhile, in the perpendicular magnetic recording type disk drive, as shown in FIG. 5A, it is known that a non-linear distortion is generated as a reproduction saturation characteristic in a GMR element of the read head. A waveform distortion, therefore, exists in the read signal waveform output from the read head. In the data reproduction operation, as described above, the read signal waveform is converted into a differential waveform by the differential circuit. At that time, as shown in FIG. 5C, there occurs a phenomenon called bit shift that a peak position of the differential waveform shifts. In short, the bit shift due to a distortion of the read signal waveform occurs in the perpendicular magnetic recording type disk drive. When the bit shift which is beyond an allowable range occurs, this can be a factor of occurrence of a read error that data can not be normally restored.
Incidentally, since the non-linear distortion of the read head occurs as the vertical asymmetry property of the read signal waveform in the longitudinal magnetic recording type disk drive, the bit shift phenomenon due to this distortion does not have to be taken into consideration.
FIG. 6 is a view showing the relationship between an output (amplitude) of a read signal during the read operation and the bit shift (peak shift). That is, although a quantity of bit shift differs depending on an output value of the read signal, this quantity exponentially drops at a given output value or smaller values (threshold value 60). In other words, in the MR reproduction saturation characteristic 50 of the read head shown in FIG. 5A, when a response range (range of amplitude values) becomes large, the influence of the non-linear distortion is increased. Conversely, when the response range is small (when 0 is a center, a range in the vicinity thereof), the characteristic can be considered as a substantially linear characteristic. To sum up, in a data bit string of a data signal to be processed (read/write data), it is confirmed that the bit shift is apt to occur in the read signal waveform (differential waveform) when its bit interval is long. Here, usually, assuming that a bit “1” is a reference, the bit interval is determined by a number of bits “0” generated between this bit and a next bit “1”. In other words, when a data signal has a data pattern which is a 3T or above pattern, the bit shift is apt to occur in the read signal (differential waveform). On the other hand, in the case of a pattern which is less than the 3T pattern, it is confirmed that the bit shift is contrary hard to be generated.
Basically, in the perpendicular magnetic recording type disk drive, since there occurs the bit shift due to the non-linear distortion generated in the read signal waveform output from the read head, a write compensation facility is required in order to solve this problem.
As a prior art, there is proposed a compensation method by which an inversion position at which generation of the non-linear bit shift is expected is detected in advance and this inversion position is shifted in the reverse direction of the non-linear bit shift (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2000-207704). Further, in order to eliminate the bit shift due to remaining magnetization in the history, there is proposed a compensation method by which a demagnetization function is provided to the head (for example, see Jpn. Pat. Appln. KOKAI Publication No. 7-134804, U.S. Patent Publication U.S. Pat. No. 5,963,385, and U.S. Patent Publication U.S. Pat. No. 6,038,093). Furthermore, there is also proposed a compensation method which eliminates the bit shift due to magnetization in the history by performing direct-current deletion immediately before recording (for example, see Jpn. Pat. Appln. KOKAI Publication No. 11-86213).
However, all of the above methods are methods for compensating the bit shift due to recording. Therefore, as described above, it is impossible to compensate the bit shift generated based on the reproduction characteristic of the read head and the bit interval of the data bit string of the data signal.